As a light source for liquid crystal displays, light sources of a backlight type in which light is irradiated from a backside of a display and a light reflective film described in Japanese Unexamined Patent Publication No. 2003-160682 are heretofore widely used because they have an advantage that they are low-profile and can illuminate uniformly. When they are used, it is necessary to place a light reflective plate on the backside of a screen to prevent the escape of illuminating light to the backside of a screen. Since a small thickness and a high reflective property of light are required for this light reflective plate, white films whitened by containing fine air bubbles within the film and reflecting light at the interface of the air bubble are principally used as a light reflective plate.
Formation of the fine air bubbles is achieved by dispersing a non-compatible polymer having a high-melting point finely in a film base material, for example, polyester, and stretching (e.g., biaxial stretching) the resulting film. Voids (air bubbles) are formed around this non-compatible polymer particles during this stretching, and the void exhibits an effect on light reflection, and therefore the film can be whitened and a high reflectance can be attained (Japanese Examined Patent Publication No. 8-16175). Light reflected off the reflective plate is diffused and light other than light having upward directivity is reflected by a prism, and the reflection is repeated between the reflective plate and the prism, and light is ultimately sent to a liquid crystal cell in a state in which the directivity of light is enhanced. In this case, if reflection efficiency of the reflective plate is low or there are factors which cause light leakage or light decay in a system, since optical loss is produced during repetition of the reflection to deteriorate energy efficiency, the brightness of a screen is lowered or economy is deteriorated.
Further, the light reflective plate is requested to reflect all color (wavelength) of light uniformly from the viewpoint of color reproducibility of a display.
Furthermore, to prevent yellowing of the film due to ultraviolet rays emitted from a cold cathode ray tube, a white film prepared by laminating an ultraviolet absorbing layer is proposed (Japanese Unexamined Patent Publication Nos. 2001-166295 and 2002-90515).
In these white films, various methods for improving properties on the brightness are disclosed. For example, for the purpose of improving the brightness in an edge light type, a method in which a light-shielding layer is provided on the film surface opposite to a light source is disclosed (Japanese Unexamined Patent Publication No. 2002-333510). Further, a method, in which light diffuseness is controlled by selecting a difference in refractive indexes between a spherical particle and a binder and front brightness is improved by a light diffusion sheet, is disclosed (Japanese Unexamined Patent Publication No. 2001-324608). Further, a method, in which brightness unevenness of a backlight is improved by controlling the diffuseness of a film surface on the light source side in a reflective sheet in a direct backlight, is disclosed (Japanese Unexamined Patent Publication No. 2005-173546).
As one method for enhancing the reflection efficiency of the light reflective plate, there is a technique in which number of voids per unit area is increased by increasing a thickness of the white film. We tried this technique, but the brightness of a display was not improved as much as expected and it was found that the wavelength dependency of spectral reflectance varied. That is, light reflectance at a long wavelength was improved, but improvement in a light reflectance at a short wavelength was small.
It could therefore be helpful to provide a white polyester film for a light reflective plate which can attain high brightness when it is used in a side light type liquid crystal display or a direct backlight type liquid crystal display.